1. Field of the Invention
The present invention relates to a printing apparatus that ejects ink from a print head to print a print medium, and a relevant printing method, and specifically, to a printing apparatus that controls the temperature of the print head, which is increased by ejection of the ink, and a relevant printing method.
2. Description of the Related Art
With the recent prevalence of computers and the Internet, printing apparatuses such as printers, copiers, and facsimile machines have spread rapidly to offices and general homes as output instruments that output images. Main printing methods adopted for these printing apparatuses include an electrophotographic method, an ink jet method, and a thermal method. Printing apparatuses adopting the ink jet method (ink jet printing apparatuses) can print media composed of various materials such as clothes, corrugated fiberboards, earthenware, and metal. The ink jet printing apparatus can also print not only planar media but also media with recesses and protrusions or a curved surface and edges of the media. The ink jet printing apparatus is thus commonly applied not only to personal use but also to business use. Furthermore, the ink jet printing apparatus has various advantages; the ink jet printing apparatus allows the size of a print head to be relatively easily reduced, allows high-resolution images to be printed at high speeds, requires low running costs, is quiet during printing, and can print color images using a simple configuration.
Currently known print heads mounted in the ink jet printing apparatus use electrothermal converting elements (heaters) or electromechanical converting elements such as piezo elements. Print heads of a type that utilizes thermal energy to eject ink are manufactured by making electrothermal converters and electrodes on a board by means of a semiconductor manufacturing process and then forming liquid path walls, top plates, and the like. This makes it possible to relatively densely manufacture individual print elements (also referred to as nozzles or ejection ports) that allow ink to be ejected as droplets. As a result, the apparatus can be made more compact. Moreover, the print head utilizing thermal energy to eject ink is excellent in ejection response frequency and suitably meets the recent demand for fast printing of high-resolution images. The print head is generally configured to include a plurality of densely integrated and arranged print elements (nozzles) each comprising a liquid path through which ink is supplied and an electrothermal converting element that subjects the ink in the liquid path to film boiling.
However, since the print head having the plurality of densely arranged print elements allows the electrothermal converting elements provided in the respective print elements to rapidly generate heat and thus energy for ejection, repeated ejections cause heat to be accumulated in the print head. The accumulated heat may pose a problem.
For example, an increasing quantity of accumulated heat raises the temperature of the ink in a common liquid chamber that temporarily stores the ink to be supplied to each of the liquid paths. A dissolved gas thus precipitates in the ink. Moreover, small bubbles generated in the individual liquid paths grow gradually as more heat is accumulated. Bubbles soon appear in the common liquid chamber, which is in communication with the liquid paths. Then, these large bubbles obstruct the feeding of the ink from the ink tank to the common liquid chamber or from the common liquid chamber to each liquid path as well as the ejection of the ink through ejection ports in the respective print elements. When an insufficient amount of ink droplets are ejected through the individual print elements even with the application of a voltage pulse to the electrothermal converting elements in accordance with image data, this condition is generally called non-ejection. In the ink jet print head, an excessive increase in temperature may result in non-ejection to degrade image quality or damage the print head. Such a heat accumulation problem more frequently occurs when an attempt is made to achieve faster printing or to output images of a higher resolution. This is because in this case, the print head with the ejection ports densely arranged therein is driven at a high frequency.
A method is known which, to avoid possible defects resulting from the above-described heat accumulation problem, detects the temperature of the print head during printing and compares the detected temperature with a predetermined threshold to control printing operations. For example, if the detected temperature is higher than the threshold, the driving frequency (ejection frequency) of the print head may be reduced to inhibit heat accumulation. For a serial type ink jet printing apparatus that forms an image by intermittently repeating main scanning of the print head and sub-scanning of a print medium, an effective method is to set a standby time for the start of the next print scan to allow the print head to cool down (Japanese Patent Laid-Open No. 2002-355959).
A method has also been disclosed which estimates the temperature of the print head expected to be detected after the completion of the next main scan according to a current temperature of the print head and the data of the image printed by the next main scan, and sets the standby time according to the estimated temperature (Japanese Patent Laid-Open No. 2001-113678).
The above-described methods make it possible to inhibit the temperature of the print head from increasing excessively. Thus, the above-described methods make it possible to avoid disadvantageous unstable ink ejections that affect image quality or damage the print head.
However, the method of controlling the driving frequency of the print head as described above can inhibit the print head from increasing excessively but poses a new problem described below. That is, when the method is adopted which reduces the driving frequency (ejection frequency) if the detected temperature is higher than the threshold, the speed of the print head relative to a print medium needs to be also reduced in association with the driving frequency of the print head. Performing control such that the driving frequency and relative speed are thus changed within the same page requires the provision of a complicated driving mechanism configuration as well as a driving control circuit. This disadvantageously increases the costs of the whole apparatus.
In contrast, no very complicated configuration is required for the method of setting the standby time between the scans as shown in Japanese Patent Laid-Open Nos. 2002-355959 and 2001-113678. However, this method has been found to affect images printed with the standby time set between each print scan and the succeeding print scan. Specifically, disadvantageously, the density or hue of an area printed with the standby time set between the print scans may be different from that of the other areas. The difference may be perceived as density or hue unevenness on the image.
Now, explanation will be given of the cause of a phenomenon such as the density or hue unevenness which may occur in an ink jet printing apparatus using multipass printing. The print head mounted in the ink jet printing apparatus has a plurality of print elements densely arranged therein. Ink is ejected through the individual print elements in accordance with image data. However, the plurality of print elements may vary to some degree in connection with the process of manufacturing the print head. Consequently, it is difficult to set all the print elements to eject exactly the same amount of ink in exactly the same direction. When an image is printed by one print scan performed by the print head with the varying print elements, the ejection characteristics of the individual print elements are significantly reflected in the image. Thus, image defects such as stripes and unevenness are visually perceived. The multipass printing method is adopted to reduce such image defects.
With the multipass printing method, image data that can be printed by the print head during one main scan is divided into a plurality of pieces. An image is formed by a plurality of main scans with a predetermined amount of sub-scan sandwiched between the main scans. That is, the multipass printing method uses a plurality of print elements to form a line that can otherwise be printed by one print element during one main scan, during a plurality of main scans. Thus, the adverse effects, on this line, of the ejection characteristics of each print element are reduced; the ejection characteristics of the print elements are distributed. This makes extreme stripes or unevenness unlikely to occur in the image as a whole. Therefore, a smooth image can be obtained.
The multipass printing method allows the number of passes to be optionally set. For example, a multipass printing method for two passes divides image data that can otherwise be printed by one main scan into two pieces so that the corresponding image is formed by two main scans. A multipass printing method for N passes divides image data that can otherwise be printed by one main scan into N pieces so that the corresponding image is formed by N main scans. The increased value of N reduces the adverse effects of the ejection characteristics of one print element on one line, smoothing the entire image.
With the multipass printing method, the density of the printed image varies depending on whether or not duration varies among the N main scans. If two droplets of ink are applied to same position on a print medium, how the first and second ink droplets applied to the print medium permeate the print medium varies depending on how the first ink droplet permeates the print medium when the second ink droplet is applied to the print medium. Thus, the density of the print image varies depending on the level of permeation of the ink. That is, the standby time between each main scan and the next main scan makes the density or hue of a part of the image printed by these print scans different from that of the other parts of the image. This phenomenon occurs differently depending on the type of the ink applied or the type of the print media applied. However, the difference in duration has been found at least to affect the image density or hue.
Thus, with the method of varying the standby time between the print scans depending on whether the detected temperature of the print head exceeds the threshold, only the area printed and scanned after the elapse of many standby time periods exhibits a density and a hue that are different from those of the other areas; this is disadvantageously viewed as unevenness. This problem may occur not only in the case of multicolor printing but also in monochrome printing. That is, in the case of monochrome printing, density unevenness may occur. In the case of multicolor printing, density and hue unevenness may occur.